This invention relates to a ditcher of the type used to excavate pipeline trenches and the like.
A conventional ditcher generally comprises a carrier, usually a crawler tractor, carrying a rotatable digging wheel at its rear. More particularly, the carrier supports an upstanding, rigid suspension frame. This suspension frame includes one or two substantially vertical beams at its rear end. Each vertical beam carries a slide guide. The digging wheel is held in a rigid wheel frame, which is generally rectangular in shape and extends around the wheel. The front end of this wheel frame is pivotally attached to a slide member. In turn, the slide member is mounted on the beam slide guide and can slide up and down thereon to alter the elevation of the wheel. A substantially vertical hydraulic cylinder is suspended from the upper end of the suspension frame and attached at its lower end to the slide member. Contraction and expansion of this cylinder moves the wheel frame between the elevated travelling position and the lowered digging position. A cable is attached to the rear end of the wheel frame. It extends over a sheave, carried by the suspension frame, to a drum on the carrier. This cable functions to suspend the wheel frame horizontally when the ditcher is digging a trench. It also serves to rotate the wheel frame upwardly, about its front pivot connection, to the elevated travelling position. The tractor is equipped with a motor, and suitable hydraulic systems and drives for operating the digging wheel and actuating the lifting cylinders. It will be noted that the conventional ditcher in use today relies on the sheer weight of the digging wheel to obtain penetration into the soil -- although it has been suggested by others that the cable may be replaced with a hydraulic cylinder to force the wheel down (see U.S. Pat. Nos. 3,226,856 and 3,510,970).
At the present time there is a need for a ditcher which can operate in the Arctic to excavate trenches for the laying of large-diameter pipelines. Existing ditchers are not suitable for this service, as will be recognized from the following discussion.
Many areas of the Arctic are covered with permafrost. This is a hard mixture of ice and organic material, such as peat. The permafrost may extend from a depth of several feet to several thousand feet. At its surface, the permafrost is frequently covered with a surface mat of vegetation, such as grass or lichen. This surface mat may vary in thickness from a few inches to a foot or more. In summer, the surface mat insulates the permafrost from the heat of the Arctic sun. If the surface mat is not present, the permafrost will, of course, melt in the summer. An example will serve to bring out the seriousness of this thermal erosion problem: A trench having a width and depth of one foot, and length of twenty feet was cut into the permafrost about five years ago. Today this trench has a width of about 20 feet, a length of several hundred yards, and a depth of approximately 50 feet.
In pipeline practise, it is necessary to excavate a trench having a level bottom. Otherwise the pipe will lie against the wall of the trench, which is undesirable. It is therefore conventional to precede the ditcher with a bulldozer which cuts a level right-of-way across side hills and the like. This practise is unacceptable for the Arctic, as the surface mat will be removed. It is possible to replace a strip of mat having a width of a few feet with wood chips -- however it is impractical to do this across a width of approximately twenty feet. In any case, the permafrost is so hard that a bulldozer cannot shave it down in a reasonable period of time.
With the foregoing comments in mind, it is one object of this invention to provide a ditcher having the combined capabilities of being able to: (1) force the digging wheel into the hard permafrost and hold it there while trenching; and (2) tilt the digging wheel, when the ditcher is operating on a side hill, so that the trench walls are always vertical and its bottom is horizontal.
As described below, hydraulic cylinders or their mechanical equivalents are preferably used in this invention to hold the wheel frame and its associated wheel in place so that this wheel assembly cannot ride up out of the trench when operating in hard permafrost. There is a liklihood that the wheel will periodically encounter particularly hard material when digging and, if rigidly connected to the hold down cylinder, it will exert excessive forces on the cylinder, eventually damaging it. In addition, in passing over bumps in the hard terrain, vertical misalignment will occur between the carrier and the digging wheel assembly, thereby also creating a strain on the hold down cylinder.
It is therefore another object of this invention to provide a limited amount of "play" or float capability at the connection between the hold down cylinder and the wheel frame so that, while the cylinder does constrain the wheel frame substantially in the digging position, a limited range of movement of the wheel frame about its pivot connection with the suspending frame is still permitted. This "play" serves another function, as explained below, in that it makes it possible to tilt the wheel frame.
The Arctic ditcher is required to excavate a uniquely large trench through extremely hard material. With this in mind, it is another object of the invention to provide a ditcher which is powered by twin engines -- one of which drives the digging wheel only while the other engine drives the remaining hydraulic components. In this manner, the power available to the digging wheel is not diminished due to periodic heavy load conditions affecting other parts of the machine.
The ditcher is called on to operate under extremely severe weather conditions. Temperature below -60.degree. F are not uncommon. Under these conditions, the frequency of mechanical breakdown is enormously increased. It is therefore another object of this invention to incorporate an emergency cross-over system into the hydraulic circuitry of the machine so that, in the event that one engine breaks down, the other engine can take over sufficient functions to enable the ditcher to lift its wheel, move off the right of way, and return to the base camp for repair, or continue operating at a reduced production rate.
It is another object of the invention to provide a ditcher having an engine -- hydraulic circuit system which can be warmed up without load on starting the machine.
The carrier's tracks must be capable of bracketing the wide trench which is excavated -- yet the carrier should still be capable of being loaded on a conventional truck bed. It is therefore another object of the invention to provide a carrier having tracks which can be spread apart or brought together, even when the machine is standing on the ground.
It is another object of the invention to provide a ditcher having a digging wheel drive which is designed to reduce the high-stress input between the sprocket and wheel segment teeth, thereby reducing the liklihood of failure at low temperatures due to brittleness of the metal.
It is another object to provide a ditcher having a braking system to enable it to operate successfully on down-hill terrain.
It is another object of the invention to provide the digging wheel buckets with teeth whose digging angle is substantially greater than that of conventional ditchers, thereby improving the excavating rate to a surprising extent.